Tweeter for coaxial mounting in midrange horn

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I'm shopping for a pair of tweeters. I'd like to try coaxially mounting them in the mouth of a mid horn, so I'd like something that is small, relative to the wavelengths it will be blocking. It would ideally have no wasted area - that is, I'd like it to be all driver & horn. If there is a mounting plate, I'd prefer it to be plastic that I can easily trim (or grind) away.

I'm interested in any suggestions and / or any finished projects like this.

Ideally, it would be happy in this configuration:

direct radiator bass
mid horn
crossovers 400-500Hz and 3-5kHz
~95dB system sensitivity

I'm currently considering drivers that have integral plastic horns:

1) P-Audio PHT407N

-used in the Signature Boominator (DSP crossed just above 2kHz)
-could be trimmed to about 80mm (cutout diameter is 75mm)
-locally available

2) Morel CAT 378

-better quality?
-very robust
-fairly small to start with; could be trimmed to about 80mm (cutout diameter is 75mm)

3) Tymphany BC25SC06-04

-cheap
-light (easy to mount)
-tiny, could be trimmed to about 50mm (cutout diameter is 46mm)

Note1: I'd also treat the back of the tweeter to reduce reflections, but I'd prefer the tweeter to be small enough to be 'invisible' to the mid.

Note2: Currently using a JBL 2445J + (OEM version of) Fostex FT17H Horn Super Tweeter, cossover at 8kHz (mini DSP). The FT17H is a relatively big unit, with a tiny horn and high crossover requirement, but still works OK sitting inside the mid horn. To my ears this is better than with tweeter on top... possibly because my main system uses coaxials, so its the set of compromises that I've trained myself to hear as 'normal'.

Note3: the horn is nominally 90*40 dispersion. Its an off-the-shelf plastic horn that I've extended to 44*57cm mouth, 36cm long.

Works well to 10kHz with a good compression driver:
http://www.diyaudio.com/forums/multi-way/288925-4-jbl-clone-diaphragm-identification.html
 
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What kind of directivity do you want to get? What is the coverage pattern of the midrange horn? Does it have to go loud?

HF directivity: the 60 conical of the P-Audio horn would be fine. Anything roughly in line with the mids will probably do.

Mid coverage: 90*50 nominal. A bit narrower in practice.

Loud: nothing crazy, this is for domestic use. The 20 watt amp I'm using for mid + HF is more than enough.

I added a note to post 1, but can't add attachments, so here be pictures.

1) Stock horns. These were 30*45 mouth, 30cm long. They had horizontal pattern control to ~900Hz, according to my dodgy polar plots (theory says ~600). They should maintain pattern control a bit lower now, since I've expanded the mouth.

2) FR plots.
Black: cone driver, 300Hz high pass, HF boost to make it "full range". In break up after 7kHz.

Green: standard JBL 2445J, same settings as the black plot. Interestingly it is only more sensitive >1kHz. In break up after 7kHz.

This is with the driver on the floor, the horn just plonked on top, mic at horn mouth.

Purple: cone driver, with crossover points set to 300Hz and 4.5kHz. Nice.

To get rid of the 4.5kHz notch would require heavy equalisation or a rebuild with a phase plug ...so I think I'll just accept that as the crossover point.

3) A modded horn, on a dual 15" box. The mods were achieved with scrap wood and filler. It needs lots of TLC + sanding to get it looking perfect ...or just a coat of matt black paint and a grill. I'm thinking I might use a grill of perforated steel, so it can double as a mount for the HF driver.
 

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HF directivity: the 60 conical of the P-Audio horn would be fine. Anything roughly in line with the mids will probably do.

3) A modded horn, on a dual 15" box. The mods were achieved with scrap wood and filler. It needs lots of TLC + sanding to get it looking perfect ...or just a coat of matt black paint and a grill. I'm thinking I might use a grill of perforated steel, so it can double as a mount for the HF driver.

Is it poor taste to quote yourself?

It occurred to me some dipoles, such as the Bohlender Graebener drivers, are exactly this - a grill of perforated steel.
 

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Updates / experimental results:

The ~4kHz notch on the cone + mid horn was an easy (acoustic) fix.

I tested a heavy grill of perforated steel. Firing the mid horn through it has marginal effect on the response.
- Therefore mounting an AMTPRO-4, in the horn's mouth, should be fine. With the face plate removed, this driver is an open frame, so the mids should fire through it as if it wasn't there.

I just ordered a couple. Am looking forward to this.

AMT tweeters - explored by Critofur - Techtalk Speaker Building, Audio, Video Discussion Forum

The FR looks OK bare, and a little smoother with a tiny "waveguide".

fetch


This is the smaller one, with "45 degree chamfer "waveguide" on 3/4" baffle"

fetch
 
Got the pro4 AMTs. They work + measure like they should, with flat, clean response (no notches or other eq needed).

The face plate came straight off. It looks + works fine without it, so my test rig has one faceless AMT plonked on a little plinth just in front of the mid horn.

I measure better extension that what Critofur shows, possibly due to me using a larger mic distance - the further the mic is from the membrane, the less path length difference between different parts of the membrane, so less cancellation (is my guess).

First impressions are good, but after an hour or three of listening, they seem louder / more silbilant than they measure (again, possibly cos I listen further back than I can easily position the mic), so I think they will take a while to get set up perfectly.

Stuff to adjust: delay, level & physical stuff such as mounting position and treatment to reduce reflections off the frame. I might wrap the back and sides in a felt 'sock'.
 
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